Film or coating deposition and powder formation

ABSTRACT

The present invention relates to film or coating deposition and powder formation.

This invention relates to film or coating deposition and powderformation.

Ceramic, polymer and other films, coatings and powders are used in, forexample, structural and functional electronic applications.

As background, the distinction between a film and a coating is veryloosely defined and is not important to the operation or description ofthe present invention. In one definition, a film would be considered asa layer under about 10 μm thick, and a coating would be of greaterthickness. In the present description, however, the terms are generallyused interchangeably.

The following are examples of previously proposed techniques forgenerating such films, coatings and powders: physical vapour deposition(PVD) (a generic term given to a variety of sputtering techniques suchas radio frequency (RF) sputtering, reactive magnetron sputtering andion beam sputtering); flame spray deposition (FSD); the so-calledsol-gel process; electrostatic spray pyrolysis (ESP); and chemicalvapour deposition (CVD).

However, none of these techniques has been found to provide good controlof the stoichiometry morphology, microstructure and electricalproperties of multicomponent oxide films and a relatively high growthrate and deposited area of a deposited film. Also, the techniques tendto need expensive equipment and highly skilled technicians for effectiveoperation.

This invention provides a method of depositing a material onto asubstrate, the method comprising the steps of:

-   -   (a) feeding a material solution comprising one or more precursor        compounds, a solvent and a pH-modifying catalyst to an outlet to        provide a stream of droplets of the material solution.    -   (b) generating an electric field to electrostatically attract        the droplets from the outlet towards the substrate; and    -   (c) providing an increase in temperature between the outlet and        the substrate.

This invention also provides apparatus for depositing films on asubstrate. the apparatus comprising:

-   -   (a) an outlet for providing a stream of material solution        droplets, the material solution comprising one or more precursor        compounds, a solvent and a pH-modifying catalyst;    -   (b) means for generating an electric field to electrostatically        attract the droplets from the outlet towards the substrate; and    -   (c) a heater for heating the substrate and providing an increase        in temperature between the outlet and the substrate.

Further respective aspects of the invention (to which the variouspreferred features are equally applicable) are defined in the appendedclaims.

Embodiments of this method, which will be referred to hereinafter aselectrostatic spray assisted vapour deposition (ESAVD), enable thefabrication of both thin and thick films. The technique combines theadvantages of CVD and electrostatic spray deposition whilst alleviatingthe problems associated therewith. In comparison to other filmdeposition techniques, ESAVD has a high deposition rate and efficiency,and allows easy control of the stoichiometry and microstructure of thedeposits. In addition, it is a simple, cheap, and low-temperaturesynthesis method suitable for the fabrication of a variety of differentfilms. The method also allows the deposition of a film on large surfacearea substrates.

The use of the pH-modifying catalyst (which can be acid or alkali) canprovide a clearer solution with increased electrical conductivity, andso can give finer droplets and thus a better coating quality.

The method can be performed in so that the substrate and other pieces ofapparatus are open to the surrounding ambient atmosphere, the otherambient gaseous reactants refer to any other gaseous reactants (such asoxygen, for example) that may be present in the atmosphere. In anotherembodiment, the method may performed within the confines of a containerand said other ambient gaseous reactants may be supplied to saidcontainer, thereby to enable the deposition of a particular film.

Both simple and multicomponent ceramic oxide films have been fabricatedusing the above mentioned method. In one embodiment, the film may be aceramic film such as PZT (Lead Zirconate Titanate) or a doped film suchas YSZ (Yttria Stabilised Zirconia). Other films may include PbTiO₃,BaTiO₃, La(Sr)MnO₃, NiO—YSZ, SnO₂—In₂O₃ and other Indium-Tin Oxidefilms. The film may also be a structural and/or functional film such asan electroceramic film.

Preferably, the droplets are charged to a voltage within the approximaterange 5-30 kilovolts with respect to the substrate.

In one embodiment, the temperature increases gradually to a temperaturein the approximate range 100 to 650 degrees Celsius (the temperatureused may depend on the type of coating). Varying the processingparameters enables the production of dense/porous and/or thin/thickfilms all of which have good adhesion to the substrate.

Preferably, the film has a thickness between a nanometre andapproximately 100 micrometers, or much thicker.

In any of the above embodiments, the catalyst may be an acid such asethanoic acid or hydrochloric acid. In this case, the required pH may bebetween 2 and 5.

Alternatively, the catalyst may be an alkali such as NH₃. In this case,the required pH may be between 9 and 12.

The invention can be particularly useful for producing polymer coatings,in which case it is preferred that the electric field is maintainedduring at least part of the time during which the substrate cools downafter coating has been performed. This can urge the polymer into adesirable polar structure.

The apparatus may further comprise a syringe pump or a static pressurefeed to provide a constant stream of coating solution to said outlet.

The apparatus may also comprise a container capable of enclosing atleast said substrate and said outlet, such that other gaseous reactantsmay be supplied for reaction with said coating solution.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 illustrates schematically an apparatus for use in electrostaticspray assisted vapour deposition of a film on a substrate;

FIG. 2 is a flow chart that illustrates schematically steps in thesynthesis of coating solution for the electrostatic spray assistedvapour deposition of YSZ;

FIG. 3 schematically illustrates the principle of electrostatic sprayassisted vapour deposition of a film from a coating solution using theapparatus of FIG. 1;

FIG. 4 schematically illustrates another embodiment of apparatus for usein electrostatic spray assisted vapour deposition;

FIG. 5 is a flow chart illustrating steps in a polymer depositionprocess;

FIGS. 6 a and 6 b illustrate X-ray diffraction patterns for polymerfilms produced by two process variants;

FIGS. 7 a and 7 b illustrate transmittance infra-red spectra for polymerfilms produced by the two process variants;

FIGS. 8 a and 8 b illustrate surface reflectance infra-red spectra forpolymer films produced by the two process variants;

FIGS. 9 a and 9 b are schematic diagrams showing dipole orientation inpolymer films produced by the two process variants;

FIG. 10 schematically illustrates a third embodiment of apparatus formaterial deposition;

FIG. 11 illustrates a fourth embodiment, used for powder deposition;

FIG. 12 (curves a and b) show x-ray diffraction patterns for thenano-powders produced at 500° C. and 800° C. respectively; and

FIGS. 13 and 14 show the microstructures of YSZ nanopowders at differentreaction temperatures.

In a first embodiment a coating solution is deposited to form a ceramicfilm on a substrate.

A film deposition apparatus as shown schematically in FIG. 1 comprisesan outlet (e.g. a nozzle, spray or similar) 5 connected to a highvoltage DC source 7 preferably variable in the range 0-30 kV. Inoperation, a voltage of between 5 and 30 KV would be normally used forthe apparatus as shown. A substrate holder 4 is earthed and is heated bya heater 2. The temperature of the substrate holder 4 is controlled bythe controller 1 and an attached thermocouple 3.

Heating the substrate holder also causes a corresponding heating of thearea surrounding the substrate 14 and between the substrate holder andthe outlet 5. This heating serves to set up a temperature gradientwhereby the ambient temperature increases as the substrate is approachedfrom the direction of the outlet. This increasing temperaturefacilitates a chemical vapour reaction (see FIG. 3) of the coatingsolution that enables deposition of the ceramic film.

When an electric field of sufficient strength is applied to the outlet5, a corona field is produced from the tip of the outlet 5. A ceramiccoating liquid is used to form the films and is fed into the outlet 5 inthe direction indicated by an arrow 6.

The outlet's inner diameter can vary from 1 mm (millimeter) to 0.1 mm.This relatively large inner diameter reduces the chances of cloggingproblems when high viscosity solutions are passed through the outlet 5.

A substantially constant flow in the range of 0.4-60 ml/h (millilitresper hour) is maintained by use of a syringe pump or a constant staticpressure feed.

In this way, the electrostatic field set up between the charged outlet 5and the earthed substrate holder 4 serves to guide charged coatingsolution droplets to the substrate 14. Droplets of the coating solutionare provided with a positive charge by way of the high voltage DC source7. These positively charged droplets are attracted to the earthedsubstrate holder 4. (As an alternative, of course, the droplets could benegatively charged with an earthed holder 4, or vice versa).

FIG. 2 schematically illustrates steps in the preparation of one type ofcoating solution for the deposition of YSZ (Yttria Stabilised Zirconia).First, a precursor compound (in this case, Zr(OC₄H₉)₄) is mixed with asolvent (in this case, Butanol—C₄H₉OH). This solution is stirred and asecond precursor compound Y(O₂C₄H₁₅)₃ (more generally, a metal alkoxideor an organometallic compound) is added under action of heat. Themixture is then catalysed to form a coating solution of the desired pH.In this case, ethanoic acid (CH₃COOH) is used as a catalyst, but otheracids (such as HCl) or alkalis (such as NH₃) may be used in thepreparation of alternative coating solutions. In the case of acidcatalysed reactions, the desired pH may be between 2 and 5. In the caseof alkali catalysed reactions, the desired pH may be between 9 and 12.

The coating solution, a mixture of Zr(OC₄H₉)₄, Butanol and Y(O₂C₄H₁₅)₃,is then passed to the outlet 5 and discharged towards the substrate 14.

Preferably, the concentration of the desired coating solution isapproximately 0.01-0.5 mol/l (moles per litre). In addition, the coatingsolution may have a viscosity in the region of about 0.01 to 50 mPa·S(millipascal seconds) depending on the type of film that it is desiredto produce.

Table 1 shows the compositions of coating solutions for variousdeposited films.

TABLE 1 Further Film 1st Precursor 2nd Precursor precursors SolventCatalyst PZT Pb(CH₃CO₂)₂ & Ti(OC₃H₇)₄ CH₃OCH₂CH₂OH Ethanoic AcidZr(OC₃H₇)₄ PbTiO₃ Pb(CH₃CO₂)₂ Ti(OC₃H₇)₄ CH₃OCH₂CH₂OH Ethanoic AcidBaTiO₃ Ba(CH₃CO₂)₂ Ti(OC₃H₇)₄ CH₃OCH₂CH₂OH Ethanoic Acid SnO₂—In₂O₃In(NO₃)₃•xH₂O SnCl₂ Ethanol Ethanoic Acid La(Sr)MnO₃ La(NO₃)₃•xH₂O &SrNO₃ 20% H₂O and about Ethanoic Acid Mn(NO₃)•6H₂O 80% ethanol YSZY(O₂C₈H₁₅)₃ Zr(OC₄H₉)₄ Propanol or Butanol Ethanoic Acid YSZ Y(O₂C₈H₁₅)₃Zr(OC₃H₇)₄ Propanol or Butanol Ethanoic Acid NiO—YSZ Ni(NO₃)₂•6H₂O &Y(O₂C₈H₁₅)₃ Propanol or Butanol Ethanoic Acid Zr(OC₃H₇)₄ CGOCe(NO₃)₃•6H₂O Gd(NO₃)₃•6H₂O 20% H₂O and HCl or HNO₃ (cerium gadolina)80% alcohol LSCF La(NO₃)₃•xH₂O Sr(NO₃)2 Fe(NO₃)₃•9H₂O 20% H₂O and HCl orHNO₃ (La_(1-x)Sr_(x)Co_(1-y)Fe_(y)O₃) Co(NO₃)₂•6H₂O 80% alcohol

In table 1, the composite precursors with alkoxide precursors areso-called “sol” precursors¹. The precursor compounds are mixed inrelative quantities appropriate to the desired stoichiometry of thedesired film. Sufficient catalyst is added to give the coating solutionthe required pH. The term Sol-gel is used to describe chemical processesin which polymeric gels are formed from metallo-organic startingsolutions (see for example: “Sol-gel Science” by C. Jeffrey Brinker andGeorge W. Shearer, published in 1990).

Ethanoic Acid is a preferred catalyst to provide a clear solution, animproved solution conductivity and therefore finer spray droplets.However, other acids and/or alkalis such as hydrochloric acid, ammonia,nitric acid, Lewis acid or sodium hydroxide would all be suitablecatalysts.

Acid or base catalysts can influence both the hydrolysis andcondensation rates and the structures of the condensed products. Acidsserve to protonate negatively charged alkoxide groups, enhancing thereaction kinetics and eliminating the requirement for proton transferwithin the transition site. Therefore, acid-catalysed condensation isdirected preferentially towards the ends rather than the middles ofchains, resulting in more extended, less highly branched polymers.Alkaline conditions produce strong nucleophiles via deprotonation ofhydroxo ligands. Base-catalysed condensation (as well as hydrolysis)should be directed toward the middles rather than the ends of chains,leading to more compact, highly branched species. Hence, if porous filmsof good quality (e.g. adhesion and porosity) are required, alkalis arepreferred as catalysts.

Similarly, various other inorganic or organic solvents can be used suchas alcohol, water, or a mixture of alcohol and water could be used.

FIG. 3 schematically illustrates the principle of electrostatic sprayassisted vapour deposition of a film from a coating solution.

The temperature preferably increases, on passing from the outlet to thesubstrate, from about 100° C. to between 400 and 650° C. At point (I) onFIG. 3, the coating solution forms a corona spray, the droplets of whichare charged to a positive potential. As the droplets are attracted tothe substrate 14 they begin to form clusters together (shown at point(II) in FIG. 3) under the influence of an increased ambient temperature.At point (III) in FIG. 3, the clusters decompose and/or react in closeproximity to the substrate to form the desired ceramic film. Theclusters may also react with other gaseous reactants such as oxygen. Forexample, the hydrolysis/condensation reaction for the production of aSol→gel transition is as follows:

Hydrolysis: ≡M-OR+H₂O⇄≡M-OH+ROH

Condensation: ≡M-OH+RO-M≡⇄M-O-M≡+ROH

-   -   ≡M-OH+HO-M≡⇄≡M-O-M≡+H₂O

Where M is the desired metal film element (in this case of YSZ,Zirconium) and R is C_(n)H_(2n+1), e.g. C₄H₉. In this method, thechemical reaction proceeds forwards (towards the right hand side of theabove equations) and the reaction time decreases, with increasingtemperature. Thus, the hydrolysis/condensation reaction is speeded upwith increasing temperature.

The substrate and other pieces of apparatus are open to the surroundingambient atmosphere, and so the other ambient gaseous reactants refer toany other gaseous reactants (such as oxygen) that may be found in theatmosphere. In another embodiment the technique may performed within theconfines of a container, and any desired ambient gaseous reactants (suchas hydrogen sulphide, for example) may be introduced into thatcontainer. These introduced gases may react with the clusters to formparticular films (such as sulphide or nitrite films, for example) on thesubstrate.

To summarise, during ESAVD, droplets of coating solution are charged andthen transform into clusters or fine particles between the dischargeoutlet 5 and the substrate 14. This transformation occurs under theaction of a corona field and an increasing temperature towards thesubstrate. These clusters and fine particles are then attracted to thesubstrate by virtue of the induced electric field. The temperaturegradient is such that the clusters and other gaseous reactants coexistaround the substrate 14. The precursor clusters undergo decompositionand/or chemical reaction with gaseous reactants just OD or in very closeproximity to the substrate surface. Chemical reactions involving coatingprecursor clusters cause the formation of the desired ceramic film—asillustrated in FIG. 3.

As this method operates on a principle whereby charged droplets from theoutlet 5 are attracted towards a grounded substrate, it is particularlysuitable for scanning or writing large surface areas and is notrestricted to particular chamber sizes as in CVD and PVD. Growth ratesachievable with this method were found to be between 0.1 and 5 micronsper minute depending upon the concentration and flow rate of coatingsolution. Higher growth rates are possible by further altering thedeposition conditions.

The achievable microstructure, grain size, composition, surfacemorphology and thickness of ceramic film are strongly dependant on theprocess conditions. The grain size in the deposited film is mainlydetermined by the droplet size, and the flow rate, viscosity andconcentration of coating solution and substrate temperature. Forexample, the gain size of ceramic films increases and uniformity of thegrain distribution decreases as the droplet sizes, flow rates,concentration, substrate temperature and viscosity of coating solutionincrease. Similarly, droplet sizes are mainly determined by the coronafield intensity and coating liquid conductivity. The mean droplet sizedecreases with increasing coating liquid conductivity. Thus, films with,nanosize grained microstructure can be deposited with the ESAVDtechnique.

The crystal phase structures of the deposited ceramic films are mainlydetermined by the temperature of the substrate 14. Ceramic films formedat lower substrate temperatures, have an amorphous or nanocrystallinecrystal phase. These films may then be treated by an additionalsintering process, to transform the crystal structure from an amorphousor nanocrystalline structure to the desired ceramic phase. A highersubstrate temperature during deposition results in an increase incrystallinity of the ceramic film deposited thereupon.

Other films, such as simple oxide films, multicomponent oxide films(e.g. PZT (Lead Zirconate Titanate—Pb(Zr_(x)Ti_(1-x))O₃), PbTiO₃,BaTiO₃, Indium Tin Oxide or La(Sr)MnO₃) or doped films (e.g. YSZ (YttriaStabilised Zirconia—(ZrO₂)_(0.92)(Y₂O₃)_(0.08)) or Ni—YSZ) etc,structural and/or functional films such as electroceramic films,nanostructured films, and/or of course films other than ceramics, may beproduced with this technique.

Referring back to FIG. 1, because the temperature gradient generated bythe projections of the substrate holder 14 towards the outlet 5 is notnecessarily constant in front of the substrate 14 in the plane of thesubstrate, the uniformity of coating thickness can be improved byrotating and/or translating (in general, moving) the outlet and/or thesubstrate holder during deposition to vary the relative positions of theoutlet and the substrate with time. If rotary motion is used, this couldinvolve, for example, rotating the substrate (which might bethree-dimensional) about an axis passing through the substrate, orpossibly rotating the outlet (or outlets, if more than one is used)about art axis which is not coaxial with the outlet's axis (i.e.“circling” the outlet around).

In another feature, the polarity of the electric field applied betweenthe outlet and the substrate holder can be reversed from time to timeduring the deposition process. This can be beneficial to avoid theaccumulation of charges (which can counteract the effect of the appliedfield), thus allowing thicker coatings to be produced.

“Graded” coatings can be produced by varying the concentration and/orcomposition of the precursor solution during deposition. (Simply, thiscan be achieved by depositing the contents of a first container (bottle)of precursor solution, and then switching to another container and soon).

Similarly, it has been found that films may be produced of a thicknessvarying from a nanometre to approximately 100 micrometers in thickness(or much thicker). The coatings can be used in microscale circuitry orfor much bulkier items such as turbine blades for jet engines, byscaling up the apparatus (the apparatus of FIGS. 1 and 4 as shown has asubstrate diameter of about 20 mm).

Single crystal substrates can be used to obtain oriented or epitaxialfilms. A range of microstructures including epitaxial, columnar andequiaxial growth are possible by varying the processing conditions.

The substrates may be conductive (e.g. metal) or non-conductive (e.g.class, polymer or ceramic).

Another embodiment of an electrostatic spray assisted vapour deposition(ESAVD) apparatus is schematically illustrated in FIG. 4.

The apparatus of FIG. 4 is similar to that of FIG. 1, except that ashaped substrate holder 104 projects towards the outlet 5 at either sideof the substrate 14′. The substrate holder 104 is heated as before, andthis heating serves to set up a temperature gradient whereby the ambienttemperature increases as the substrate 106 is approached from thedirection of the outlet 107. (The arrangement of FIG. 1 also provided anincrease in temperature approaching the substrate 14 from the directionof the outlet 5, but the arrangement of FIG. 4, with the projectingparts of the substrate holder 104, provides a more gradual temperaturegradient). This increased temperature and more gradual temperaturegradient facilitates solvent evaporates and decomposition of the coatingsolution near the vicinity of the substrate that enables deposition ofthe film.

The use of the apparatus of FIG. 4 or indeed, FIG. 1) to produce PVDFpolymer coatings on the substrate will now be described.

The improvement of the performance of polymer films, and the ability tofabricate specific bulk polymer with tailored surface compositions forparticular application have become important considerations. The needfor designing polymer with well-controlled chemical compositions at thesurface arises from the fact that interfacial phenomena defineproperties that are crucial to the service performance of a particulardevice. Examples of applications where polymer surface properties areimportant include wetting, printing, biomedical and electronic devices.In all these cases, molecules from the “environment” approach thepolymer surface and experience interfacial forces due to electrostaticand positive/or negative charged cloud interactions. It thus becomespossible, in principle, to design guided approaches towards deviceoptimization by controlling polymer film growth in order to alter orcontrol interfacial interactions of polymers by the provision ofappropriate chemical structure in the surface layers.

Surface modification techniques have been used widely in polymerindustry. The techniques of flame treatment, acid etching, and coronadischarge treatment after the fabrication of the bulk polymer materialshave been used extensively in industrial applications, produce a varietyof new polar surface. This is quite acceptable in wetting and printingapplications, but for applications of polymers in electronic andbiomedical devices. the presence of a polar surface is not sufficient.The alignment of polar groups in polymer films along preferredorientation need to be considered as well. Plasma surface treatmenttechniques have the advantage for the surface modification of commoditypolymer substrates, but the penetration depth of the treatment is verylow at a reaction level for useful surface modification. Meanwhile, theequipment of plasma technique is very expensive and needs highly skilledtechnicians.

A typical example for the polymer depositions is to fabricate thepiezoelectric and pyroelectric polyvinylidene fluoride (PVDF) film. PVDFfilm has a large dielectric constant, due to the large dipole moment ofCF₂, and is one of the most polar dielectric polymers. Its advantagesover ceramic materials include light weight, flexibility, toughness,ease of fabrication and low permittivity. The conventional fabricationof PVDF films for electronic application normally involves twoproduction steps. Firstly, PVDF bulk films are produced by aconventional method such as cast, hot pressing, dipping and spin coatingof PVDF solution. Then, PVDF bulk films are treated by the modificationtechniques such as high thermal high voltage poling, corona poling,stretching and electron beam discharge etc. However, the deposited PVDFfilms cannot be stretched, it is difficult to prepare β-phase crystalfilm by this method. On the other hand, the breakdown of thin filmsoccurs easily under a high electric field. As a result, it is not easyto pole thin PVDF films. In the present technique, the film productionand surface modification poling of polymer PVDF films can be achieved ina single production step.

Therefore, in this application, the outlet's inner diameter preferablyreduces towards the outlet's tip from about 1 mm to about 0.1 mm. Thisrelatively large initial inner diameter ensures that the likelihood ofclogging problems with high viscosity solutions (such as PVDF) in theoutlet 5 is significantly reduced.

A substantially constant flow of PVDF coating liquid to the outlet 5 ismaintained by use of a syringe or pneumatic pump (not shown).Preferably, the solution flow is in the range of 0.4-10 ml/h.

In this way, the electrostatic field set up between the charged outlet107 and substrate holder 104 serves to guide charged coating solutiondroplets to the substrate 14′. As described earlier, droplets of thecoating solution are provided with a negative charge by way of the highvoltage DC source 1. These negatively charged droplets are thusattracted onto the substrate and in moving towards the substrate passthrough a region of increasing temperature gradient. The temperaturegradient ensures that the solvent evaporates before the PVDF precursordroplets reach the substrate 14′ and the chemical reaction occurs juston or in very close vicinity of the substrate surface to form a PVDFcrystal phase film coating.

After the PVDF film coating has been applied, the syringe pump can beturned off and heating can be stopped. The PVDF film coated substrate isthen cooled down up to room temperature, with the electric fieldmaintained during this cooling process.

FIG. 5 is a flow chart illustrating steps in a polymer depositionprocess.

Referring to FIG. 5, a precursor solution used to deposit the films ofPVDF comprises a mixture of poly vinylidene fluoride (PVDF) powder andsolvent N,N-Dimethylformamide (DMF) or N,N-Dimethylacetamide (DMA). As afirst step, PVDF powders are dissolved in DMA or DMF solvent. Thissolution is stirred and heated at 60° C. for thirty minutes. A clearsolution with 0.01 to 0.01M concentration in PVDF is yielded. Then,acetic acid C₃COOH is added into the solution as catalyst according topH 2 to 5 and conductivity range greater than or equal to 2.0 us at roomtemperature. After thirty minutes stirring, a clear precursor solutionfor PVDF film coating is obtained.

Although there is no complete agreement among investigators regardingthe mechanism responsible for piezoelectricity and pyroelectricity inPVDF, there is nearly unanimous agreement that a polar crystal form isrequired for optimum activity. One of the complicating factors aboutPVDF is that it can exist in four different crystal forms. The crystalphase in which the chain conformation is trans-gauche-trans-gauche iscalled α-phase. The chains are then packed in a monoclinic unit cellwhich is non-polar.

A variety of techniques have been employed to form PVDF films into asecond crystal phase called β-phase in which the chain conformation isessentially all trans and the chains crystallized in an orthorhombicunit cell with a net dipole moment.

It is thus known that there are at least two stable crystal forms ofPVDF, a planar zig-zag polar form (α-phase) and T-G-T-G non-polar form(β-phase). One can obtain an oriented β-form from the α-form in PVDFfilms by mechanical stretching or rolling, corona discharge, and hightemperature high voltage poling. A number of researchers have shown thatthe β-form is very important in obtaining good piezoelectricity andpyroelectricity in PVDF films.

Since the β-form crystal exhibits a net dipole moment, the presentprocess uses the inventors' observation that molecular dipoles on chainswithin the crystalline regions of the polymer become aligned with theapplied electric field during the poling process and are then relativelystable in such an orientation in the absence of the piezo- and pyroelectric response. Only the crystalline regions of the polymer wouldbecome permanently aligned and then only that component of the chainaxis which lies in the plane of the film would be expected to contributeto the polarisation.

For identifying the influence of corona field in trials of this process,in fact two experimental processes were compared. In the first case, thecorona field was turned off while samples were cooled down to roomtemperature after deposition (process I). In the second case, the coronafield was maintained while samples were cooled down to room temperatureafter deposition (process II), to maintain the dipole orientation of thedeposited material in a required orientation until the materialsolidified.

X-ray diffraction patterns of the PVDF film produced under process I andprocess II are shown in FIGS. 6 a and 6 b respectively. The diffractionpeak observed at 2θ≈20.8° is assigned to unresolved (110) and (200)diffraction of β-phase in PVDF. α-phase in PVDF shows diffraction peaks2θ≈18° assigned to (100), (020) and (021) respectively. The comparisonof process I with process II shows that the intensity of main peak atabout 20.8° increases under corona field. It indicates that some partsof α-phase have transformed into β-phase in PVDF. This result wasconfirmed by respective infra-red (IR) spectra, as shown in FIGS. 7 a(Process I) and 7 b (Process II) for transmission spectra, and FIGS. 8 a(Process I) and 8 b (Process II) for reflectance spectra.

Many journal papers have reported on the crystal forms of PVDF from IRspectra. The α- and β-phase crystal forms have many common absorptionband characteristics (such as CH, CH₂, CF, CF₂, and C—C etc.) asreported in the literature. It is known that the crystal forms can beidentified by the characteristic absorption bands of β-phase at about510 and about 1280 cm⁻¹, and that of α-phase at about 530, about 610 andabout 795 cm⁻¹.

From the results of IR analysis between process I and II, it is clearthat the contents of β-phase is higher in the PVDF film produced usingprocess II, but a certain amount of α-phase may still exist underprocess II. It indicates that PVDF films prepared by process II consistmainly of the β crystal phase with some α-phase, meanwhile, it is alsofound that corona field strongly influenced the surface structure ofPVDF film. From the spectra, it is observed that since the absorptionpeak at about 1280 cm⁻¹ in process II is assigned to the CF₂ symmetricstretching vibration of β crystal phase, and is stronger than that inprocess I, the results of the IR spectra suggest that β-form crystal isoriented and the CF₂ dipoles are aligned along the applied corona field.

There have been many investigation of α-phase and β-phase crystal inPVDF film. it is well known that the α-phase crystal is more stable thanthe β-phase crystal. The reason why the β-phase crystal was formed inspite of its instability was not clear. In the present case of ESAVD, itis proposed that the stability and the mechanism of β-phase crystalformation is as follows. Because β-phase crystal is the polar crystal,β-phase is stabilized and formed in preference to α-phase when thecorona field exist during the deposition of PVDF film. Consequently, thecontent of β-phase increases with increasing the energy supplied to thesubstrate by the corona discharge. Under the conditions shown in theexperimental section, the charge droplets of the PVDF solution wereattracted onto the substrate and the PVDF film was formed by evaporationof the solvent and decomposition of precursor solution during substratetemperature field. PVDF polymer seems to have enough mobility to changethe conformation aligned along the applied field direction under theexistence of the corona field during evaporation of the solvent. But ifthe corona field does not exist after deposition (as process I), acertain amount of energy is obtained to rearrange the PVDF moleculesbecause the substrate temperature is high enough near/or over PVDFmelting point 170° C. In contrast, when the corona field exist duringcooling down of the PVDF film to room temperature, the polar groups inPVDF film are “cooled” along the applied corona field direction.

FIGS. 9 a and 9 b are schematic diagrams showing dipole orientation inpolymer films produced by the two process variants, process I andprocess II. These illustrate that under process II a PVDF film withoriented β-phase crystal, which is very important for getting goodpiezoelectricity and pyroelectricity in PVDF film.

The present studies thus show that the oriented thin PVDF film can beprepared directly onto a substrate in single step by a novel ESAVDtechnique. The corona field is maintained during substrate cooling toform oriented polar polymer film PVDF, but other forms of PVDF can beproduced without maintaining the field. The corona field helps totransport the charge droplets of PVDF solution onto the substrate toform PVDF thin film, and forces the polar group in PVDF thin film toalign along the corona field.

In conclusion, the utilization of corona field in the vapour depositionprocess is effective in controlling crystal forms and their orientation,to form polar polymer films.

The present results also clearly revealed the potentials of thistechnique to deposit polymer thin film of good quality with a verysimple equipment. This technique can be used in the fabrication of awide range of polymer films, including polar and conductive polymer/orco-polymer coatings, such as PVDF, PTFE, polyanilines, and polypyrroleetc.

FIG. 10 illustrates a third embodiment of a deposition apparatus. Inmany respects, the apparatus of FIG. 10 is similar to that of FIG. 4,but for the addition of deflectors 210 under the control of a deflectioncontroller 200.

The deflectors are used to deflect the spray of electrically chargeddroplets emerging from the outlet. This can steer or concentrate thespray on particular desired areas of the substrate, or move the spraydistribution around to help to compensate for an uneven temperaturegradient near the substrate. The deflectors can be electrostatic plates,in which case the deflection controller supplies a high voltage betweenthe plates to deflect the electrically charged spray emerging from theoutlet, and/or magnetic deflectors (eg a yoke coil or other winding), inwhich case the deflection controller supplies a current to coils in thedeflectors to generate a magnetic field to deflect the charged spray.

In other embodiments, the substrate could be heated to a temperaturejust below that required for film deposition (e.g. 50° C. below thatrequired for film deposition). If a laser beam is then directed onto thesubstrate by a suitable beam-steering mechanism and used to heat verylocalised areas of the substrate while the solution is being sprayedfrom the outlet, deposition will occur selectively at those areas of thesubstrate.

In further alternative embodiments, masking can be used to mask offcertain areas of the substrate to give control over where the film isdeposited.

FIGS. 11 to 14 relate to the use of these techniques to generatepowders, rather than films or coatings. The example to be described isthat of YSZ powders, but many other materials (particularly thematerials described above with reference to the film or coatingtechniques) could be used.

The differentiation between film (coating) production and powderproduction, using basically the same apparatus, is mainly one oftemperature (although other operational parameters can be varied). Ifthe ambient temperature between the outlet and the substrate isincreased then the droplets of coating solution will tend to form powderparticles before hitting the substrate. This effect can be exaggeratedby slowing down the flight of the droplets—e.g. by changing the flowrate or the electric field—to give more time for the powder to form.Alternatively, if a cold substrate is used, then the droplets willcondense into powder particles on hitting the substrate.

FIG. 11 illustrates a suitable apparatus for manufacturing the powderedYSZ. The apparatus comprises a tubular, up-flow reactor equipped with anexternal resistive heater. YSZ sol precursor is delivered at anappropriate flow into a stainless steel capillary outlet (100 μm insidediameter, 650 μm outside diameter) which is maintained at a dc voltageof 10-30 kV (positive polarity). The capillary electrode is placed 15-30mm from a ring electrode maintained at 1 kV dc voltage. The function ofthe ring electrode is to focus the spray aerosol into the reactorchamber and select the appropriate nanosized particles to reach thesubstrate. The high local electrostatic field at the capillary tipcauses an emission of charged aerosol from the highly deformed liquidinterface. A stream of Argon gas from a coaxial glass capillary (800 μminside diameter) carries the spray charged aerosol through the ringelectrode into the reaction zone of the reactor, finally to arrive atthe cold substrate (having a negative polarity).

The YSZ sol precursor with monodispersed nanosize particles issynthesized as follows. First, Y(NO₃)₃.6H₂O is dissolved in 2-propanolwhile stirring for about 15 min at room temperature yielding a clearsolution. Zirconium tetra n-butoxide. Zr(OC₃H₉)₄ is added into theprevious solution according to the stoichiometry of a desired finalcomposition of (ZrO₂)_(0.92)(Y₂O₃)_(0.08). The final sol precursorconcentration and pH are preferably 0.05M and 3-5 respectively. Thehydrolysis and condensation can be carried out at room temperature understirring.

The YSZ nanoparticles produced by a prototype of this technique wereanalysed using x-ray diffraction and transmission electron microscopy(TEM).

The quality of the YSZ nanopowders depends strongly on the processconditions. Below a reaction temperature of 450° C., all samplesappeared to be amorphous from the x-ray diffraction traces. FIG. 12(curves a and b) shows x-ray diffraction patterns for the nano-powdersproduced at 500° C. and 800° C. respectively. The x-ray diffractionpatterns show that fully stabilized cubic zirconia (YSZ) nanopowders aredirectly formed. The presence of monoclinic or free Y₂O₃ phases were notdetected in YSZ powders produced at about 500° C. No other new phaseswere observed. It indicates that Y₂O₃ has been perfectly dissolved intothe ZrO₂ lattice to form a solid solution.

FIGS. 13 and 14 show the microstructures of YSZ nanopowders at differentreaction temperatures. In the nanopowders formation process, the YSZaerosol is produced by electrostatic assisted spray, and delivered intoreaction zone in CVD reactor chamber, the fine droplets of aerosolchange into dry gel and pyrolysis to form the nanopowders onto coldsubstrate under an appropriate low temperature. TEM micrographs revealthat the distribution of YSZ nanoparticles is uniform and the averagesize of YSZ powders deposited at 500° C. is 10-20 nm (FIG. 13). Underthe high reaction temperature (e.g. 800° C.), particle-clusteraggregation occur. This is because aerosol droplets changed intoparticles at high reaction temperatures. TEM micrograph reveals that thedistribution of YSZ nanoparticles is not uniform and the YSZ particlessize is in the range of 30-80 nanometres at high temperatures (FIG. 14).

1. A method of depositing a material onto a substrate, the methodcomprising the steps of: (a) feeding a material solution comprising oneor more precursor compounds, a solvent and a pH-modifying catalyst to anoutlet to provide a stream of droplets of the material solution, (b)generating an electric field to electrostatically attract the dropletsfrom the outlet towards the substrate; and (c) providing an increase intemperature between the outlet and the substrate.
 2. A method accordingto claim 1, in which step (b) comprises: applying a voltage to theoutlet such that droplets of the material solution emerging from theoutlet are charged and attracted to the substrate by virtue of theelectric field.
 3. A method according to claim 1 or claim 2, comprisingthe step of relatively rotating and/or translating the outlet and thesubstrate during coating deposition.
 4. A method according to any one ofthe preceding claims, comprising the step of varying the materialsolution composition and/or concentration during the coating process. 5.A method according to any one of the preceding claims, comprising thestep of reversing the polarity of the electric field between the outletand the substrate at intervals during the deposition process.
 6. Amethod according to any one of the preceding claims, comprising the stepof locally heating areas of the substrate to enhance material depositionat the heated areas.
 7. A method according to any one of the precedingclaims, comprising the step of electrostatically and/or magneticallysteering the stream of droplets in transit from the outlet to thesubstrate.
 8. A method according to any one of the preceding claims,wherein the material is deposited as a film.
 9. A method according toclaim 8, wherein the film is a multicomponent oxide film; a simple oxidefilm or a doped film.
 10. A method according to claim 8 or claim 9,wherein the film is one or more of a structural film: a functional film;and an electroceramic film.
 11. A method according to any one of claims1 to 7, in which the material is deposited as a powder.
 12. A methodaccording to any one of the preceding claim, in which the materialsolution is a polymer solution.
 13. A method according to claim 12,comprising the step of maintaining the applied electric field for atleast part of the time during which the material deposited on thesubstrate is allowed to cool.
 14. A method according to any one of thepreceding claims, wherein the catalyst is an acid, added in sufficientquantity to give a material solution pH of between 2 and
 5. 15. A methodaccording to claim 14 wherein the catalyst is selected from the groupconsisting of: ethanoic acid and hydrochloric acid.
 16. A methodaccording to any one of claims 1 to 13, wherein the catalyst is analkali, added in sufficient quantity to give a material solution pH ofbetween 9 and
 12. 17. A method according to claim 16 wherein thecatalyst is NH₃.
 18. A method according to any one of the precedingclaims, wherein the droplets of material solution are charged toapproximately 5-30 kilovolts with respect to the substrate.
 19. A methodaccording to any one of the preceding claims, wherein the temperatureincreases to a temperature in the approximate range from about 100 toabout 650 degrees Celsius.
 20. A method according to any one of claims 1to 18, wherein the temperature increases to a temperature in theapproximate range from about 100 to about 400 degrees Celsius.
 21. Amethod according to any one of the preceding claims, wherein the methodis performed within the confines of a container and the other ambientgaseous reactants are supplied to the container, thereby to enable thedeposition of a particular film.
 22. A method according to any one ofclaims 1 to 11, wherein the material is Lead Zirconate Titanate (PZT),and the material solution is manufactured by the steps of: (a) mixingCH₃OCH₂CH₂OH (solvent) with a first precursor compound Pb(CH₃CO₂)₂ andZr(OC₃H₇)₄ and a second precursor compound Ti(OC₃H₇)₄, and (b) adding acatalyst to the mixture to provide a material solution of a required pH.23. A method according to any one of claims 1 to 11, wherein thematerial is PbTiO₃, and the material solution is manufactured by thesteps of: (a) mixing CH₃OCH₂CH₂OH (solvent) with a first precursorcompound Pb(CH₃CO₂)₂ and a second precursor compound Ti(OC₃H₇)₄, and (b)adding a catalyst to the mixture to provide a material solution of arequired pH.
 24. A method according to any one of claims 1 to 11,wherein the material is BaTiO₃, and the material solution ismanufactured by the steps of: (a) mixing CH₃OCH₂CH₂OH (solvent) with afirst precursor compound Ba(CH₂CO₂)₂ and a second precursor compoundTi(OC₃H₇)₄, and (b) adding a catalyst to the mixture to provide amaterial solution of a required pH.
 25. A method according to any one ofclaims 1 to 11, wherein the material is SnO₂—In₂O₃, and the materialsolution is manufactured by the steps of: (a) mixing ethanol (solvent)with a first precursor compound In(NO₃)₃.xH₂O and a second precursorcompound SnCl₂, and (b) adding a catalyst to the mixture to provide amaterial solution of a required pH.
 26. A method according to any one ofclaims 1 to 11, wherein the material is La(Sr)MnO₃, and the materialsolution is manufactured by the steps of: (a) mixing about 20% H₂O andabout 80% ethanol (solvent) with a first precursor compoundLa(NO₃)₃.xH₂O and Mn(NO₃).6H₂O and a second precursor compound SrNO₃,and (b) adding a catalyst to the mixture to provide a material solutionof a required pH.
 27. A method according to any one of claims 1 to 11,wherein the material is Yttria Stabilised Zirconia (YSZ), and thematerial solution is manufactured by the steps of: (a) mixing propanolor butanol (solvent) with a first precursor compound Y(O₂C₈H₁₅)₃ and asecond precursor compound Zr(OC₄H₉)₄, and (b) adding a catalyst to themixture to provide a material solution of a required pH.
 28. A methodaccording to any one of claims 1 to 11, wherein the material is YttriaStabilised Zirconia (YSZ), and the material solution is manufactured bythe steps of: (a) mixing propanol or butanol (solvent) with a firstprecursor compound Y(O₂C₈H₁₅)₃ and a second precursor compoundZr(OC₃H₇)₄, and (b) adding a catalyst to the mixture to provide amaterial solution of a required pH.
 29. A method according to any one ofclaims 1 to 11, wherein the material is NiO—YSZ, and the materialsolution is manufactured by the steps of: (a) mixing propanol (solvent)with a first precursor compound Ni(NO₃)₂.6H₂O and Zr(OC₃H₇)₄ and asecond precursor compound Y(O₂C₈H₁₅)₃, and (b) adding a catalyst to themixture to provide a material solution of a required pH.
 30. A methodaccording to any one of the preceding claims, wherein the film has athickness between a nanometre and approximately 100 micrometers. 31.Apparatus for depositing films on a substrate, the apparatus comprising:(a) an outlet for providing a stream of material solution droplets, thematerial solution comprising one or more precursor compounds, a solventand a pH-modifying catalyst; (b) means for generating an electric fieldto electrostatically attract the droplets from the outlet towards thesubstrate; and (c) a heater for heating the substrate and providing anincrease in temperature between the outlet and the substrate. 32.Apparatus according to claim 31, comprising a syringe pump to provide astream of material solution to the outlet.
 33. Apparatus according toclaim 31 or 32, comprising a container for enclosing at least thesubstrate and the outlet, such that other gaseous reactants may besupplied for reaction with the material solution.
 34. Apparatusaccording to any one of claims 31 to 33, comprising a heatable memberdisposed, at least in part, in a region between the substrate and theoutlet, to provide, when heated, a temperature gradient between theoutlet and the substrate.
 35. Apparatus according to any one of claims31 to 34, comprising one or more electrostatic and/or magneticdeflectors for deflecting the path of the droplets between the outletand the substrate.
 36. A method of depositing a material onto asubstrate, the method comprising the steps of: (a) feeding a materialsolution to an outlet to provide a stream of droplets of the materialsolution, (b) generating an electric field to electrostatically attractthe droplets from the outlet towards the substrate; and (c) providing anincrease in temperature between the outlet and the substrate.